1. Field of the Invention
The invention relates to a punching tool for punching out parts of any desired shape from paper, cardboard, pasteboard, plastics material foils, leather, rubber and the like.
2. Description of the Prior Art
Punching tools of the type that is known, for example, from DE-PS 33 17 777 Cl, are preferably used in strip steel punching tools for punching out parts of any desired shape from flat materials, in particular folding box blanks. They are increasingly also used in rotational punching tools for the same purposes.
The punching tools must fulfill three conditions: first, the material must be reliably and completely separated over the entire punching surface, so that no damage to the product occurs in the subsequent operating steps; second, the cut piece must come away perfectly with very little dust and fiber and with smooth cut edges; and, thirdly, the cutting friction should be as small as possible, this being shown in the number of good punching operations. Using blades of the type that exist at present, these three requirements can only be achieved in an inadequate manner.
The cutting edges of the known punching tools are usually produced by grinding, scraping, and, at present, by means of lapping, based on the principle that the cutting edge should be as sharp as possible. Cutting edges which are significantly below 0.010 mm are achieved, but these have an uneven microsaw-like contour and, owing to the production, the smallest grinding burrs become attached thereto. These cutting edges must first be equalized and smoothed, in alternating action with the cutting material, such that, subsequently, the best possible cutting quality can be obtained. Depending on the cutting quality, several thousand punching operations may be necessary for this.
These very narrow cutting edges are exceptionally sensitive to mechanical loads, in particular to pressure. A particular significance is attached thereto because the usual operating method with this punching technology of the above-mentioned type is that of steel on steel, i.e., the blade cutting edges operate against a reinforced printing plate.
The punching lines have, for reasons of production, height differences of +/-0.02 mm. In addition, further height tolerances are generated from the punching machines, the tool production and the handling of the punching processes.
In inexpedient cases, all the mistakes accumulate, such that height differences of up to 2 mm cannot be considered impossible.
In order to guarantee that the complete separation of the stamping material is ensured, the height differences must be compensated in such a manner that the very sensitive cutting edges are not damaged by overloading.
In practice, the punch is subjected to pressure until 50% of all cutting points are separated. Subsequently, the remaining differences in height are equalized by means of subsequent adhering with suitable strips of different strengths. Depending on the box blank, up to 100 m of punching lines may be found in one strip steel stamping mold, as a result of which the equalization of the height differences known in the technical field as "preparation" requires much care and experience and takes many hours during which the punching machine is inactive. These preparation times are, in some cases, longer than the subsequent actual production time. These times when the machine is inactive cause unproductive additional costs. Frequently this leads to the punches being brought together so far that the largest part of the punching line separates, but at the expense of a large part of the cutting edges being damaged or destroyed. Under the excess pressure, the cutting edge is deformed in an undefined manner. This causes a cut which is not clean, in the case of which dust and stamping hairs form to a greater extent and also leads to cardboard covers and rear parts tearing. The working life of cutting tools of this type is substantially reduced, such that the tools must be replaced prematurely, this causing correspondingly increased costs.
Many attempts have been made to make it possible to simplify the preparation or to replace it altogether. There is known, from DE 31 35 980 Cl, a strip steel punching tool of the initially-mentioned type, in which punching lines with deformable blade backs are used for automatic height equalization. This solution fails as regards the technical possibilities in the production of the intended deformation places on these punching lines.
DE 39 28 916 Cl, describes a process in which the cutting edges which are too high can deflect towards the rear against a soft metal plate. In this connection, the plate is work-hardened in the pressure area of the blade backs, such that a stabilized balance state sets in.
This process fails because of the constructive features of the punching machines which prevail in the market.
In the case of a strip steel punching tool which is known from DE 33 17 777 Cl, the backs of the blades are formed as reinforced cutting edges and are mounted on a softer metal plate, in which, under the action of the punching pressure, they can deflect corresponding to the height difference. The production of these lines is expensive and, in practice, demonstrates that the soft plates become unusable prematurely owing to the digging in of the hardened back cutting edges as a consequence of fatigue factors. They must therefore be exchanged relatively rapidly, as a result of which, as regards cost, the technical advantages are lost.
More recently, it has been possible, in order to increase the working life, to coat the cutting edges in a manner which reduces wearing and friction. Coatings of this type are, in general, of a ceramic type with extremely high levels of hardness, but are correspondingly brittle. In-house tests have proved that these wear-reducing layers are likewise destroyed by the damage to the cutting edge until they lose their effect.
A cutting edge blade for cutting a moving cardboard sheet is known from U.S. Pat. No. 2,349,336, the cutting edge of which cutting edge blade has a curvature with a radius of 0.05 to 0.1 mm and a cutting angle of 55.degree. to 80.degree.. As a result of this cutting shape, it is intended that the cardboard sheet which is under a tensile stress be separated, before the cutting edge has moved completely through the cardboard sheet. A punching stroke is, however, not performed using this cutting blade.